In recent years, a seat back side portion or a roof compartment portion above a door of a vehicle is provided with a side airbag for protecting the head of passengers when the vehicle collides. The side airbag unit includes an inflator for generating expansion gas after a large impact is received due to the vehicle collisions and an airbag which is supplied with gas from the inflator and expanded. As for the storage state of the airbag before the side airbag unit is actuated, the airbag is stored in the seat back side portion of a vehicle seat such that the airbag is folded in a predetermined procedure and the side airbag unit is covered with a seat cover or the like.
Usually, the surface of the vehicle seat is covered with a seat cover made of fabric or leather in order to cover the cushion material of the seat, and the seat cover on the seat back side portion equipped with a side airbag unit has an opening for expansion through which the airbag is to be inflated. Conventionally, the expansion opening is sewed together with a weak sewing thread which is cut out easily when it is pulled with a predetermined force.
When a vehicle collides so that a large impact is received, the side airbag unit senses the impact and generates a high pressure gas from its inflator and introduces the high pressure gas into the airbag, so that the airbag is inflated in a moment. When the airbag is expanded, an excessive pulling force is applied to the sewing thread which sews together the expansion opening and consequently, the sewing thread is cut out by the force. Then, the closed expansion opening is opened and then the airbag comes out expanded from the opening. The airbag is expanded sideway of a passenger so as to support the head, chest and waist of the passenger by absorbing the shock. Consequently, the impact applied to the human body at the time of a collision is relaxed largely so as to secure the safety of the passenger.
If a configuration which closes the airbag expansion opening with the sewing thread is adopted, the seat cover needs to be placed after the airbag unit is loaded onto the seat. Consequently, manufacturing process is limited, which is an inconvenient problem. Because recent vehicle seats are equipped with various auxiliary devices such as a heater, seat level adjusting actuator in many cases, the freedom on the manufacturing process has been demanded. Further, if the airbag unit needs to be inspected after a vehicle is delivered to a customer, conventionally, the airbag unit cannot be inspected until the seat cover is removed, thereby complicating the working process.
Additionally, a breaking resistance of the sewing thread is changed largely depending on its sewed state and further changed depending on rises in temperature of a vehicle compartment or aging, which is another inconvenience. If the sewing thread is not cut out easily, the airbag may not be expanded at an emergency, which is a fatal inconvenience. If the strength of the sewing thread is low, the sewing thread is broken by a pulling force applied to the seat cover when a passenger is seated, so that the expansion opening is cleaved and the seat cushion is exposed, which damages the appearance.
When the airbag is expanded, the sewing thread is cut out one by one, so that the expansion speed of the airbag is lowered, which is still another inconvenience.
Instead of the configuration which closes the expansion opening using the sewing thread containing such various inconveniences, the closing configuration for the expansion opening using the slide fastener has attracted public attentions. The slide fastener can open and close the expansion opening by moving the slider up and down even after the seat cover is attached. Therefore, the freedom of the assembly process is intensified, so as to enable a complicated configuration seat to be assembled easily. Additionally, at the time of inspection of the airbag unit, wiring and installation portions can be inspected easily by opening and closing the slide fastener.
If coupling of coupling elements is released on part of the slide fastener in which the coupling elements are coupled with one another and a lateral pulling force is continued to be applied under this state, the coupling of the coupling elements can be released successively with a weak force without moving the slider. Therefore, different from a state in which the sewing thread is broken one by one, the cleavage velocity is higher so that the airbag can be expanded quickly so as to protect a passenger. Particularly, because a gap between the head of the passenger and the side glass of a vehicle is small, the side airbag needs to be inflated in a short time after a collision.
For example, Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2006-15158) has disclosed a slide fastener with an emergency opening means having a configuration which can be cleaved easily in order to inflate and expand the airbag.
The slide fastener with the emergency opening means described in Patent Document 1 uses insert molding method of fixing the coupling elements to the coupling element attaching portion of the fastener tape at the same time when the coupling elements are molded. The configuration of the coupling element of the slide fastener includes two leg portions which are fixed to the coupling element attaching portion on the fastener tape side edge portion such that the leg portions sandwich the coupling element attaching portion, a body portion which connects the two leg portions, a coupling head formed on the outer end portion for coupling a pair of the coupling elements, right and left in a lateral pulling direction and a neck portion which is twisted to connect the coupling head with the body portion. A groove is formed in the apex of the coupling head along the coupling axis line. Further, a shoulder portion intended to be fitted to the groove is formed in the central portion in the width direction of the coupling element such that the shoulder portion is projected from the body portion and the neck portion.
The coupling head is fitted in between the neck portions of the two adjacent coupling elements fixed to the opposite other side fastener tape in order to prevent coupled coupling element rows from being cleaved in the lateral pulling direction. The shoulder portion projecting from the neck portion and body portion of the coupling element is fitted to the groove portion provided in the opposite coupling head, so as to prevent the right and left fastener stringers from being deviated in a shearing direction and decoupled from one another.
In the slide fastener described in Patent Document 1, the rear side of the coupling head which is to be fitted to the shoulder portion of the coupling element is cut out so as to obtain a nose-like shape asymmetrical between the front and rear sides and two pieces of the coupling element are disposed in the central portion of the slide fastener. The portion is cleaved easily by a pushing force from the rear side of the slide fastener and becomes a cleavage starting point in the slide fastener when the airbag is inflated.
Patent Document 2 (Japanese Patent Application Laid-Open Publication No. 10-155524) has disclosed a slide fastener in which coupling elements are disposed at a constant interval in the shape of a rope ladder by molding two connecting threads at the same time. In the slide fastener descried in Patent Document 2, the connected coupling element rows are bent into a U shape so as to sandwich the coupling element attaching portion on the side edge portion of the fastener tape. After that, the coupling element rows are sewed to the fastener tape by placing the sewing thread such that the sewing thread strides around the leg portions of the coupling elements, so that the coupling head of the coupling element is projected outward from the coupling element attaching portion on the side edge portion of the fastener tape.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2006-15158    Patent document 2: Japanese Patent Application Laid-Open Publication No. 10-155524